This is excellent!
Several years ago I built a prototype of a video renderer on nodejs (v0.8-ish). It's a silly thing to do, rendering video in javascript (especially a decade ago), but it worked well enough to prove that it could be done and a startup pivot was born - one that was eventually acquired by Vimeo.
While a colleague (and now friend) was working on porting my silly little renderer to C/C++ to try to get closer to real-time, I built out a UI to allow us to "templatize" dynamic video for our users. This made it possible for our designers to design the video experience for the videos that were dynamically generated for our users' content.
That UI very much resembled Flash. Since then I've always wanted to do what you've done here. Before acquisition, I was asking our designers to walk me through how they use After Effects, in the hopes of building our tools in that direction, but then Vimeo showed up and... not too long after I left to start a travel startup. I haven't revisited the video space since.
I love this. I _knew_ that it could be done - especially now that WASM is stable - and I'm excited to see someone has done it. And free, no less!
Edit: Thought it was open source - thanks for the correction. Also I was way off on the node version.